1. Field of the Invention
This invention relates to amorphous metallic alloys, commonly referred to as metallic glasses, and more particularly to a new process for the preparation of amorphous metallic components and tools, particularly with high aspect ratio features (ratio of height to width greater than one) in the micro- and submicrometer scale.
2. Description of the Related Art
Amorphous metallic alloys are metal alloys that can be cooled from the melt to retain an amorphous form in the solid state. These metallic alloys are formed by solidification of alloy melts by undercooling the alloy to a temperature below its glass transition temperature before appreciable homogeneous nucleation and crystallization has occurred. At ambient temperatures, these metals and alloys remain in an extremely viscous liquid or glass phase, in contrast to ordinary metals and alloys which crystallize when cooled from the liquid phase. Cooling rates on the order of 104 or 106 K/sec have typically been required, although some amorphous metals can be formed with cooling rates of about 500 K/sec or less.
Even though there is no liquid/solid crystallization transformation for an amorphous metal, a xe2x80x9cmelting temperaturexe2x80x9d Tm may be defined as the temperature at which the viscosity of the metal falls below about 102 poise upon heating. Similarly, an effective glass transition temperature Tg may be defined as the temperature below which the equilibrium viscosity of the cooled liquid is above about 1013 poise. At temperatures below Tg, the material is for all practical purposes a solid.
Amorphous parts are typically prepared by injection casting the liquid alloy into cold metallic molds or by forming the parts in the superplastic state at temperatures close to the glass transition temperature (Tg). However, micrometer scale parts with high aspect ratios cannot be prepared by these processes. The aspect ratio of a part is defined as the ratio of height to width of the part. A part with a high aspect ratio is considered to have an aspect ratio greater than one.
Casting of a high aspect ratio part requires long filling times of the liquid alloy into the mold. However, because metallic alloys generally require high cooling rates, in an injection casting method, only small amounts of material can be made as a consequence of the need to extract heat at a sufficient rate to suppress crystallization. Moreover, cold mold casting does not enable the alloy to wet the mold effectively, thereby leading to the production of imprecise parts.
U.S. Pat. No. 5,950,704 describes a method for replicating the surface features from a master model to an amorphous metallic alloy by forming the alloy at an elevated replicating temperature. In this method, a piece of bulk-solidifying amorphous metallic alloy is cast against the surface of a master model at the replication temperature, which is described as being between about 0.75 Tg to about 1.2 Tg, where Tg is measured in xc2x0 C. However, at these temperature ranges, the alloy material is still fairly viscous. Thus, forming high aspect ratio parts is difficult because the alloy may not be fluid enough to fill the shape of the mold in a fast enough time before the onset of crystallization. Furthermore, due to the high viscosity of the alloy, high pressures are needed to press the alloy against the model.
Accordingly, what is needed is an improved method and apparatus for the formation of amorphous metallic parts, and more particularly, a method and apparatus for the formation of high aspect ratio amorphous metallic parts.
The needs discussed above are addressed by the preferred embodiments of the present invention which describe a manufacturing process that separates the filling and quenching steps of the casting process in time. Thus, in one embodiment, the mold is heated to an elevated casting temperature at which the metallic alloy has high fluidity. The alloy fills the mold at the casting temperature, thereby enabling the alloy to effectively fill the spaces of the mold. The mold and the alloy are then quenched together, the quenching occurring before the onset of crystallization in the alloy. With this process, compared to conventional techniques, amorphous metallic parts with higher aspect ratios can be prepared.
In one aspect of the present invention, a method of forming an amorphous metallic component is provided. A mold is provided having a desired pattern thereon. An alloy capable of forming an amorphous metal is placed in contact with the mold. The mold and the alloy are heated to a casting temperature above about 1.5 Tg of the alloy to allow the alloy to wet the mold. The alloy is cooled to an ambient temperature to form an amorphous metallic component.
In another aspect of the present invention, the method of forming an amorphous metallic component comprises providing a mold having a desired pattern thereon. An alloy capable of forming an amorphous metal is placed in contact with the mold, and the mold and the alloy are heated to a casting temperature wherein the viscosity of the alloy is less than about 104 poise, preferably less than about 102 poise, to allow the alloy to wet the mold. The alloy is cooled to an ambient temperature to form an amorphous metallic component.
In another aspect of the present invention, the method of forming an amorphous metallic component comprises providing a mold having a desired pattern thereon. An alloy capable of forming an amorphous metal is placed in contact with the mold, and the mold and the alloy are heated to a casting temperature above the nose of the crystallization curve of the alloy to allow the alloy to wet the mold. The alloy is cooled to an ambient temperature to form an amorphous metallic component.
In another aspect of the present invention, a method of forming an amorphous metallic component having a high aspect ratio is provided. A mold is provided having a desired pattern thereon, wherein at least a portion of the mold includes a recess having a height greater than a width thereof. The mold is filled with a metallic alloy capable of forming an amorphous metal at an elevated casting temperature, wherein the metallic alloy has sufficient fluidity to substantially fill the recess before undergoing crystallization. The alloy is cooled from the casting temperature to an ambient temperature, the cooling occurring prior to crystallization of the metallic alloy, such that an amorphous metallic component is formed replicating the shape of the mold. Components formed by this method preferably have aspect ratios greater than about one, more preferably greater than about three.